NEW ORLEANS -- Dead animals on the side of the road are an unfortunate part of American mobility, so much so that the other day, as I was driving northwest along Interstate 10 between New Orleans and Baton Rouge, it took me a while to realize the magnitude of the carnage passing before my eyes.

For a mile or more, the carcasses must have been registering only in my subconscious, as though they were telephone poles or night reflectors -- five, 10, 15, 20. Then I reached a dead-animal tolerance threshold, and the decaying lumps took on a new and tragic meaning. I saw the slaughter in its totality (death everywhere, by the score, evoking a battlefield) and in its individualism.

As the dead animals became real and thus important to me, I was struck by something else -- I had no idea what they were. They might have been muskrats, but they were too big; or small beavers, but for their rodent-like tails.

And so it was, out of nausea and concern, that I learned the improbable story of Louisiana's nutria.

Nutria is Spanish for fish otter, and if nature had been left alone -- that is a big if; it rarely is -- these creatures would not encounter many highways. Their native habitat is among the remote rivers, ponds and marshlands of Argentina and Chile. They are brown-furred, 20-pound, web-footed, herbivorous rodents. And while they do resemble beavers and muskrats, they have an exclusive place in the animal world as the only members of their family, the Myocastoridae. They also are known as coypu, Greek for mouse and beaver and the preferred designation of biologists.

Their fur, though hardly in the mink category, is of some value, and that is how and why they got to Louisiana.

In the 1930s, modest numbers of them were introduced to North America and Europe for breeding. Among the experimental breeders was E.A. McIlhenny, millionaire Tabasco Sauce baron and naturalist, who secured 20 nutria from a friend in Buenos Aires and penned them at his place on Avery Island on the Louisiana coast south of New Iberia.

McIlhenny was just starting his nutria fur farm when a hurricane struck: Who could have imagined then that the most permanent environmental effect of that storm would be the release of a small number of nutria into the marshy wild?

To say that they took to south Louisiana -- with its vast wetlands and temperate climate -- would be an understatement. From their hurricane-emancipated past, the nutria population exploded into the millions, reaching a peak of perhaps 20 million by the late 1960s. Greg Linscombe, a research biologist at the Louisiana department of wildlife and fisheries, said exponential growth of that sort is common among exotic animals when they are placed in an environment that suits their life styles and is at least temporarily void of natural enemies.

Nutrias had escaped or been released from fur farms in several parts of the country, but in most places the climate and geography conspired to kill them off after a few years. They cannot stand cold weather -- their tails have been known to freeze and fall off after a few days below 20 degrees.

But for better and worse, they thrived down here, especially in what is known as Cajun country, the geocultural trapezium whose points are New Orleans, Houma, Cameron and Lafayette. At times they have thrown the ecology out of balance, gnawing at sugar and rice crops, damaging marshes by overeating the aquatic vegetation and pushing the muskrat out of its natural habitat.

But the population has been controlled to some extent by trapping. In Louisiana, which despite its dominant oil rigs and petrochemical plants has some justification in calling itself the sportsman's paradise, trapping is not only permitted but encouraged for both economic and ecological purposes.

"People think nature controls these things," said Linscombe, the research biologist. "Well, that is true and not true. Nature will eventually control it, but the population can build to such levels that it denudes the environment before it crashes. By the time nature controls it, the damage has been done. That's why we believe very strongly in managing the population by trying to promote the fur."

While the reddish-brown nutria fur has never been popular in the United States, it developed a strong following in parts of Europe, where women use it for coats and gloves. Furriers praise the nutria coat especially for its warmth and the ease with which it can be dyed.

Because of its ability to meet Europe's nutria demand, Louisiana has emerged in recent years as the No. 1 fur-trapping state in the country. Of the 2 million wild pelts the state provides each year -- including muskrat, raccoon, bobcat, red and gray fox and beaver -- about 1.3 million are nutria. The trapping of most of those other animals is controversial even here, but for some reason no one seems too concerned about trapping the nutria.

Rare is the creature that provokes disinterest in the environmental world, but this rodent comes close. Most naturalists consider it a pest whose introduction into North America has done more harm than good.

Perhaps that is true -- for everyone but the 6,000 or so backwater Cajuns who live off the land catching nutrias and alligators. The trapping season begins in December, when the nutria fur thickens for winter, and lasts until mid-February. Entire families move out into the marsh and work together as units in large trapping camps. The children are let out of school for those two months and bring their homework with them.

The price of nutria fur has declined in the last 10 years from a high of $15 to today's $4 per pelt, prompting some of the trappers to concentrate on alligators and shrimp.

That may answer part of the riddle: Why did the nutria cross the road? It could be overpopulation in that area, or it could be that one side of the marsh was drying up for lack of rainfall. It could be that they were forced from burrows by chemicals dumped illegally on roadsides at night -- the not unheard-of practice known as "midnight dumping."

In any case, the nutrias, slow-moving waddlers not known for their brainpower, are making a mess of things on the roadside. "I don't want to sound cold about it, but it's been going on for years," said state wildlife official Dave John. "The dang things just can't get to the other side."